Induction of CD18-mediated passage of neutrophils by Pasteurella haemolytica in pulmonary bronchi and bronchioles

Pasteurella haemolytica is an important respiratory pathogen of cattle that incites extensive infiltrates of neutrophils into the lung. In addition to the parenchymal damage caused by factors released by P. haemolytica, neutr ophils contribute to the pathologic changes in the lungs. Molecules which m ediate neutrophil infiltration into the lungs during P. haemolytica pneumon ia are poorly characterized. To determine whether the CD18 family (beta(2)- integrin) of leukocyte adhesion molecules mediates initial passage of neutr ophils into the pulmonary bronchi and bronchioles of lungs infected with P. haemolytica, three Holstein calves homozygous for bovine leukocyte adhesio n deficiency (BLAD) (CD18-deficient neutrophils), and three age- and breed- matched control calves (normal CD18 expression) were inoculated with P. hae molytica A1 via a fiberoptic bronchoscope and euthanized at 2 h postinocula tion. Sections of lung were stained for neutrophils, and the intensity of n eutrophilic infiltration was determined by computerized image analysis. Sig nificantly fewer (P < 0.05) neutrophils infiltrated the lumen, epithelium, and adventitia of bronchioles and bronchi in lungs of calves with BLAD comp ared to normal calves, which had dense infiltrates within these sites at 2 h postinoculation. The reduced infiltration in calves with BLAD occurred de spite the presence of an extremely large number of neutrophils in periphera l blood that is typical for these calves. The large number of neutrophils i n the blood of calves with BLAD is probably a physiologic response that can occur without microbial colonization, since one calf with BLAD that was ra ised under germ-free conditions had large numbers of neutrophils in the blo od that were similar to those in a calf with BLAD that was raised conventio nally. Neutrophil counts in the germ-free and conventionally reared calves with BLAD were much higher than those in the three normal calves raised und er germ-free conditions. The work in this study demonstrates that during th e initial inflammatory response, neutrophils with normal CD18 expression pa ss more readily than CD18-deficient neutrophils into the walls and lumen of bronchi and bronchioles. It suggests that CD18 is needed for initial passa ge through the extensive extracellular matrix of the bronchi and bronchiole s. This has potential importance for the development of therapies to direct or inhibit neutrophil infiltration into conducting airways rather than alv eolar spaces.